Optical fibers are commonly used to transmit information using light. Often it is necessary to remove a portion of the light traveling in an optical fiber or to inject light into an optical fiber already transmitting light. Fiber couplers are used in these applications. Fiber couplers are specified by their coupling ratio, which indicates what percentage of light will be removed from, or injected into an optical fiber. Couplers with coupling ratios within the range of 0.1/99.9 to 50/50 are useful in different applications.
A common type of fiber coupler used is a biconical fused fiber coupler. Such couplers are made by contacting two or more fibers along their length and then heating a short section of the fibers while pulling the fibers lengthwise. The heated section of the fibers stretches which reduces the diameter of the fibers in the heated region. The fibers are stretched until a desired coupling ratio is achieved. The stretched portion of the fibers acquires a biconical shape. The light-carrying cores of the fibers are reduced in diameter and move closer together, facilitating coupling of light between the cores of the fibers.
Biconical fiber couplers made in this way tend to be rather long. Typically, such couplers are about 4-6 cm long. This relatively long length can be a problem in many applications because of the high cost of space in optical fiber systems. It is highly desirable to minimize the size of optical components so that costs are reduced.
Prior art methods of making fused fiber couplers produce fiber couplers having a relatively inaccurate coupling ratio. This reduces the manufacturing yield, thereby increasing the cost. Also, inaccurately manufactured fiber couplers are difficult to use in wavelength division multiplexing applications where an accurately defined coupling ratio (typically 99.98/0.02) is very important to obtain wavelength-dependent coupling characteristics.
U.S. Pat. No. 4,291,940 to Kawasaki discloses a method of making biconical taper optical fiber couplers. Couplers made according to Kawasaki will be relatively long and therefore expensive to use in optical fiber systems.
U.S. Pat. No. 4,426,215 to Murphy discloses a method of making a biconical taper fiber coupler in which the fiber cores in the tapered section are scrambled by reheating and pushing the fibers together.
U.S. Pat. No. 5,339,374 to Murphy discloses a method of making a biconical taper fiber coupler in which the fibers are heated and twisted.